You would be better getting a quote from a local firm that your neighbours have used and recommend. To give you an example; I have quite a large 3 detached house and used a local firm to do a total replacement (windows were quite deep). I had three 3 pane windows, one 1 pane, a smaller 2 pane ( toilet), a 5 window bay and four 2 pane windows for under £3,500 (double glazing- no patio windows). That was three years ago. It was a two man job, it took a week to have the windows custom made to size locally, they took the windows out and replaced them one at a time in just over two days. No mess left at all and I paid the total bill on completion of work. I didn't sign anything upfront; they trusted me and I trusted them. I had a selection of glass styles to choose from but chose the cheapest- single glass panes.
Hope that helps to give you a bit of an idea. My daughter, in a smaller house, recently got a quote from a national chain, for a laugh, it came in at £15,000. She found a local firm to do it for under £3000.